1. Field
This application relates to MOSFET circuits, exponential circuits and CDMA communication systems.
2. Background
CDMA communication systems typically include one or more base stations for communicating with various subscriber stations, such as mobile cell phones and the like. Each base station often receives a signal from several subscriber stations at the same time.
In order to minimize interference between these signals, the power level of the transmitted signal from each subscriber station is often adjusted so that its received signal strength is approximately equal to the signal strength of the other signals at the same base station. This adjustment is usually made frequently because the received signal strength often varies as the distance between the subscriber station and the base station varies.
To accomplish this, the base station typically measures the received power level, compares it to a desired power level, and delivers a signal to the subscriber station indicating the needed power level. The subscriber station receives the signal and adjusts its output power level accordingly.
A variable gain amplifier is often used in the subscriber station to implement the needed adjustment in its output power level. It is often desirable that the subscriber station output power level be adjusted exponentially or, as it is known in the art, xe2x80x9clinear-in-dBxe2x80x9d. As a consequence, the gain signal, which adjusts the output power level or power gain of the variable gain amplifier linearly, needs to vary according to the exponent of the power level signal delivered from the base station.
The exponential computation can be implemented in the digital domain. One approach is to create and refer to a table in memory that maps a set of power level signals to the needed gain signal for each. Such a map, however, requires significant memory space.
An algorithm could instead be used in the digital domain. Such an algorithm, however, would require programming, memory for the program and its operation, and processor time.
Whether using a map or an algorithm, a further problem with making the computation in the digital domain is that a digital to analog converter (xe2x80x9cDACxe2x80x9d) is often required to convert the digital output into the analog signal needed to control the variable gain amplifier. The exponential relationship between the power level signal and the need amplifier gain often requires the same incremental change in the power level signal to generate progressively smaller-incremental changes in the output power level at the lower levels of the power level signal. To insure that these very small incremental changes are accurate, a high resolution DAC is often needed, which can be expensive.
MOSFETs are also known to provide an exponential function when operated in their sub-threshold mode (also known as their xe2x80x9cweak inversionxe2x80x9d mode). Operation in the sub-threshold mode means that the range of voltages delivered to the gate of the MOSFET is below the threshold voltage of the MOSFET needed to fully turn on the MOSFET. However, this exponential function is known to vary widely as a function of the temperature of the MOSFET and as a function of structural variations that occur during the manufacture of the MOSFET. As a result, MOSFETs have not been considered to be good candidates for accurately performing the exponential function in a CDMA subscriber station.
One aspect is an amplifier that may include a gain control circuit having an input and a first MOSFET configured to operate in a sub-threshold mode to generate a gain control signal that is an exponential function of a signal applied to the input.
The operation of the first MOSFET may be affected by variation in its temperature or physical structure.
The amplifier may also include a variable gain amplifier having a gain that is controlled by the gain control signal.
The amplifier may also include a compensation circuit having a second MOSFET configured to operate in a sub-threshold mode and to compensate for the effect on the first MOSFET of variation in its temperature or physical structure.
Another aspect is an amplifier that may include means having an input and a first MOSFET for operating in a sub-threshold mode and for generating a gain control signal that is an exponential function of a signal applied to the input.
The operation of the first MOSFET may be affected by variation in its temperature or physical structure.
The amplifier may also include means for providing a gain that is controlled by the gain control signal.
The amplifier may also include means having a second MOSFET for operating in a sub-threshold mode and for compensating for the effect on the first MOSFET of variation in its temperature or physical structure.
Another aspect is a compensated exponential circuit that may include a circuit having an input and a first MOSFET configured to operate in a sub-threshold mode to generate a signal that is an exponential function of a signal applied to the input.
The operation of the first MOSFET may be affected by variation in its temperature or physical structure.
The circuit may also include a compensation circuit having a second MOSFET configured to operate in a sub-threshold mode and to compensate for the effect on the first MOSFET of variation in its temperature or physical structure.
The second MOSFET may have operational characteristics that are closely matched to the first MOSFET and may be located on the same substrate as the first MOSFET.
Another aspect is a compensated MOSFET circuit that may include a first circuit having a first MOSFET configured to operate in its sub-threshold mode, an input, and an output.
The circuit may also include a compensation circuit having a second MOSFET configured to operate in a sub-threshold mode and to compensate for the effect on the first MOSFET of variation in its temperature or physical structure.
Another aspect is a compensated MOSFET circuit that may include a first, second and third MOSFET.
All three MOSFETs may have closely matched operational characteristics, be co-located on the same substrate, and be configured to operate in their sub-threshold mode.
The circuit may also include a first variable resistor configured to communicate with the gate of the first MOSFET and having a control input configured to communicate with the second MOSFET.
The circuit may also include a second variable resistor configured to communicate with the second and third MOSFETs and having a control input configured to communicate with the second MOSFET.
The circuit may also include a current mirror circuit configured to communicate with the second and the third MOSFETs.
Another aspect is a compensated MOSFET circuit that may include a first and second MOSFET.
Both may have closely matched operational characteristics, be co-located on the same substrate, and be configured to operate in their sub-threshold mode.
The circuit may also include a resistor configured to communicate with the gate of the first MOSFET and with the drain of the second MOSFET.
The circuit may also include a constant current circuit configured to communicate with the gate and drain of the second MOSFET.
It is to be understood that other embodiments will become readily apparent to those skilled in the art from the following detailed description, wherein only embodiments are shown and described by way of illustration. As will be realized, there are many other and different embodiments, and the details that are discussed are capable of modification in various other respects, all without departing from the spirit and scope of what is claimed in this patent application. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature, not as restrictive.